The present invention is related to integrated circuits (IC), and more particularly to charge-pump circuits.
The fabrication processes for complimentary metal-oxide semiconductor (CMOS) ICs have evolved fast in the past few years for attaining higher speeds and lower power consumption. Typically, an N-channel MOSFET is fabricated by diffusing two identical N regions, called the source and the drain, side-by-side into a P-type silicon slice. A layer of insulating silicon-oxide, a.k.a. gate-oxide, is grown over the surface. A third conducting poly-silicon layer, a.k.a. the gate, is placed above the gate-oxide and between the two N-regions.
When a positive voltage is applied to the gate, charge-inversion takes place in the P-type silicon region below the gate-oxide. The charge-inversion layer extends from the source to the drain and is called the channel. Moving charge or current in the channel can be increased by increasing the gate voltage of the transistor.
Changes that have been made to the fabrication process of MOSFETs, have resulted in reduction of layer geometries, including the thickness of the gate-oxide layer. This reduction of the layer geometries, in consequence, has put a limit on maximum voltage that can be applied to the gates of transistors. If the voltage exceeds the specified limit, it causes physical damage to the gate-oxide. This phenomenon, which is referred to as oxide-breakdown, creates fissures in the gate-oxide and damages transistors.
In many analog applications where an N-channel transistor switch needs to pass a large amplitude signal (up to the supply voltage) from the source to drain, a high level voltage at the gate needs to be a threshold above the input signal in order to pass the signal through uncorrupted. This high level voltage at the gate is beyond the supply voltage and requires special circuitry, known as a charge-pump, to generate it. Unfortunately, charge-pumps are susceptible to many variations that affect their output. For example, the output of the charge-pump will vary over many variables, such as temperature, supply, clock speed, process variation, and the like. These undesirable variations may cause the output voltage to go over the absolute maximum voltage allowed by the associated fabrication process resulting in oxide-breakdown.
What is needed is a way to generate a high voltage that is constant over all variations such that the voltage does not go above the oxide-breakdown limit.
Briefly described, the present invention is directed at providing charge-pump circuit designed to generate a higher voltage than the available power supply. A feedback technique helps to maintain the voltage at a constant level in spite of power supply and temperature and process variations.
According to one aspect of the invention, a charge pump includes a feedback path that is used to help maintain a constant voltage higher than the available supply voltage.
According to another aspect of the invention a switched capacitor interface generates a target voltage that is used to activate and deactivate a bypass capacitor interface to maintain the constant voltage. The switched capacitor interface includes capacitors that are sized to create the target voltage.
According to another aspect of the invention the bypass capacitor interface is configured to complete a feedback path. The feedback helps to ensure that node n1, that is coupled to the output of the charge pump, stays at a constant potential, irrespective of the power supply voltage.
According to yet another aspect of the invention, a method is directed at generating a higher voltage than the available power supply. A comparison is made to determine if the charge pump is at the target voltage. Based on the comparison, feedback helps to ensure that the higher voltage is constant.